1. Field of the Invention
This invention relates to oil and gas production well stimulation. Particularly, the invention provides a method for stimulation of oil wells which are low producing or have ceased to produce.
2. Description of the Prior Art
Increasing problems with scale build-up in existing production and injection wells in the United States have and are being encountered. This scale build-up significantly interferes with the production from the producing wells and injection of water into the injection wells causing them to become low-producing wells or low injectors or in their ceasing to be producers or injectors at all. Generally, this scale is the result of calcium carbonate building up in the well bore, perforations, areas surrounding the well bore, tubing and other down-hole production equipment.
A conventional method for removing such scale formation is the so-called "acidizing" technique wherein an aqueous mineral acid or an aqueous mineral acid and a hydrocarbon solvent are introduced into the well bore. This aqueous mineral acid or an aqueous mineral acid and a hydrocarbon solvent may or may not be overflushed with sufficient produced water to move a portion of said aqueous acid or aqueous acid and hydrocarbon solvent back into the reservoir. The purpose of the acid is to dissolve or solubilize the carbonate scale build up so that it can be removed by flushing or swabbing. Normally scale and hydrocarbon co-precipitate. The hydrocarbon precipitation normally is in the form of paraffin or asphaltenes. There can also exist a situation wherein scale and hydrocarbons are precipitated in alternate layers. The scale can also become oil wet through the normal production of oil.
Methods for such fluidization are described in U.S. Pat. No. 4,541,483 wherein a non-aqueous solution of an oxyalkylated phosphate ester surfactant is introduced to a well and is thereafter displaced into the well formation utilizing either water or a hydrocarbon.
One of the problems encountered with the conventional acidizing treatment is that a short time after introduction of the acid and hydrocarbon material to the well, a separation occurs interfering with the dispersion of the acid in the hydrocarbon solvent. Thus, the distribution of the acid in the well bore is not uniform. Also, since the internal well elements including the scale are, in essence, "oil-wet", the acid cannot penetrate into the scale in order to solubilize it. Thus, the oil film on the surfaces of the elements and scale prevent direct contact with the acid. As results, such acidization treatments are relatively ineffective because of the inability of the acid component to remain in a dispersed form within the treating solution and further, its inability to attack all of the oil-wet surfaces of the interior elements of the well production equipment.